Scratch Match.

Solving a case like the October 2002 sniper shootings around Washington, D.C., hinges heavily on firearms identification. How do examiners match evidence to a gun? If a gun is recovered, a forensic scientist test-fires it to determine the markings it leaves on bullets and cartridge casings. The examiner then compares these under a microscope with the caliber (diameter), rifling pattern (series of grooves), and impressions and striations (microscopic marks left by unique imperfections in a gun's firing pin and barrel) on the bullets and casings found at the crime scene. If test-fires don't match, or if no gun is found, the examiner will measure the rifling patterns on the recovered ammunition and compare them with the General Rifling Characteristics database to see which gun models might match. The problem in some cases, however, is that "20 to 150 brands of firearms could leave the same rifling pattern," says Scott Doyle, forensic specialist for the Kentucky State Police in Louisville.If the examiner needs to go to the next level, he can take photographs of the impressions and striations and send them online to the National Integrated Ballistic Information Network (NIBIN), overseen by the Federal Bureau of Alcohol, Tobacco, and Firearms (ATF) and the Federal Bureau of Investigation. The sniper shootings have raised debate over whether the ATF should turn NIBIN into a national ballistic-fingerprint system. Firearms manufacturers would be required to test-fire every new gun and enter images into NIBIN. The system would help but would not be a panacea. The marks from a gun can change if the barrel rusts over time; criminals can tamper with the barrel, too, says Robert Shem, chief examiner for the Alaska Crime Lab in Anchorage. "You will always need a human expert, comparing evidence under a microscope, to say 'Yes, this bullet came from that gun."